id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
503711327,MDU6SXNzdWU1MDM3MTEzMjc=,3381,concat() fails when args have sparse.COO data and different fill values,1634164,open,0,,,4,2019-10-07T21:54:06Z,2021-07-08T17:43:57Z,,NONE,,,,"#### MCVE Code Sample

```python
import numpy as np
import pandas as pd
import sparse
import xarray as xr

# Indices and raw data
foo = [f'foo{i}' for i in range(6)]
bar = [f'bar{i}' for i in range(6)]
raw = np.random.rand(len(foo) // 2, len(bar))

# DataArray
a = xr.DataArray(
    data=sparse.COO.from_numpy(raw),
    coords=[foo[:3], bar],
    dims=['foo', 'bar'])

print(a.data.fill_value)  # 0.0


# Created from a pd.Series
b_series = pd.DataFrame(raw, index=foo[3:], columns=bar) \
             .stack() \
             .rename_axis(index=['foo', 'bar'])
b = xr.DataArray.from_series(b_series, sparse=True)

print(b.data.fill_value)  # nan


# Works despite inconsistent fill-values
a + b
a * b


# Fails: complains about inconsistent fill-values
# xr.concat([a, b], dim='foo')  # ***

# The fill_value argument doesn't help
# xr.concat([a, b], dim='foo', fill_value=np.nan)


def fill_value(da):
    """"""Try to coerce one argument to a consistent fill-value.""""""
    return xr.DataArray(
        data=sparse.as_coo(da.data, fill_value=np.nan),
        coords=da.coords,
        dims=da.dims,
        name=da.name,
        attrs=da.attrs,
        )


# Fails: ""Cannot provide a fill-value in combination with something that
# already has a fill-value""
# print(xr.concat([a.pipe(fill_value), b], dim='foo'))


# If we cheat by recreating 'a' from scratch, copying the fill value of the
# intended other argument, it works again:
a = xr.DataArray(
    data=sparse.COO.from_numpy(raw, fill_value=b.data.fill_value),
    coords=[foo[:3], bar],
    dims=['foo', 'bar'])
c = xr.concat([a, b], dim='foo')

print(c.data.fill_value)  # nan

# But simple operations again create objects with potentially incompatible
# fill-values
d = c.sum(dim='bar')
print(d.data.fill_value)  # 0.0
```

#### Expected

`concat()` can be used without having to create new objects; i.e. the line marked `***` just works.

#### Problem Description
Some basic xarray manipulations don't work on `sparse.COO`-backed objects.

xarray should automatically coerce objects into a compatible state, or at least provide users with methods to do so. Behaviour should also be documented, e.g. in this instance, which operations (here, `.sum()`) modify the underlying storage format in ways that necessitate some kind of (re-)conversion.

#### Output of ``xr.show_versions()``
<details>

INSTALLED VERSIONS                                                                                                                                                               
------------------                                                                                                                                                               
commit: None
python: 3.7.3 (default, Aug 20 2019, 17:04:43)
[GCC 8.3.0]
python-bits: 64
OS: Linux
OS-release: 5.0.0-32-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.2

xarray: 0.13.0
pandas: 0.25.0
numpy: 1.17.2
scipy: 1.2.1
netCDF4: 1.4.2
pydap: None
h5netcdf: 0.7.1
h5py: 2.8.0
Nio: None
zarr: None
cftime: 1.0.3.4
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.1
dask: 2.1.0
distributed: None
matplotlib: 3.1.1
cartopy: 0.17.0
seaborn: 0.9.0
numbagg: None
setuptools: 40.8.0
pip: 19.2.3
conda: None
pytest: 5.0.1
IPython: 5.8.0
sphinx: 2.2.0
</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3381/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue